1. Field of the Invention
This invention relates generally to electronically scanned radiometers and a method of operating these radiometers and more particularly to an apparatus and method for controlling the direction of receptivity of radiometric emissions and thereby utilizing the enhanced emission receptivity to improve the performance of the radiometer.
The invention further pertains to (a) electronically scanned arrays of radiation receptors; and (b) connection of the array to electronic systems which effect the electronic scan and processing the resultant detected radiation.
2. Description of the Prior Art
Radiometers are used in many applications to detect and process the naturally occurring emissions of objects whose temperature is above absolute zero. All objects at such temperature radiate low level electromagnetic signals across the whole frequency spectrum. Radiometers have the ability to detect these low level signals, process the signals and then display them in easily manageable form.
To display radiometrically derived signals, thereby creating a visual image of the scene in the field of view of the radiometer, requires a scanning of the radiometer's receiving beam in at least one plane. Scanning in the orthogonal plane is frequently achieved by means of the forward motion of the radiometer's support platform, for example, the forward motion of a satellite, a plane or a ship on which the radiometer is located.
In a typical example, the radiometer's receiving beam has a beamwidth of two degrees and is scanned over an angular range of .+-.40.degree.. The central frequency of observation may typically be 37 GHz with a wider band observation extending to 1 GHz on either side of the central frequency. Scanning of the receiving beam is usually achieved by means of an auxilliary mechanism in which the radiometer's antenna, which forms the receiving beam, is physically rotated. The rotation may be a full 360.degree. or extend only over the required range (in the present example +40.degree.).
It is desirable to provide an electronic alternative to mechanical scanning, through physical rotation of the antenna, since mechanical mechanisms are liable to failure especially in the harsh environments in which radiometers are usually found. Furthermore, it would also be desirable to take advantage of the flexibility provided by electronic scanning to enhance the performance of the radiometer.